The present invention relates to the display and interpretation of visual data. It particularly relates to a method and apparatus for providing an observer with pairs of images so that information is derivable from a pair which is not (or not so easily) derivable from the individual images. The information may be consciously recognised as such by the observer (e.g. information about differences between the images of a pair), or subconsciously assimilated (e.g. when the images constitute a stereoscopic pair, so that the brain derives information to create a single 3-dimensional perceived image). Of course, the conscious and subconscious processes may occur simultaneously.
It is known to provide a stereoscopic pair of images, by means of a pair of similar photographs which show the same scene from "left" and "right" viewpoints a few centimeters apart (the spacing being similar to the spacing of the eyes). The "left" photograph is presented to an observer's left eye, and simultaneously the "right" photograph is presented to his right eye to enable him to perceive not two separate images but a single, fused, image, which appears 3-dimensional. In such known techniques, care is always taken that the two photographs should be identical (except for the difference in viewpoint), and should be viewed under identical conditions by means of a pair of identical viewing systems, one for each of the observer's eyes. Thus only specially prepared pairs of photographs are usable, and an observer must view these in psychological isolation, cut off from the surroundings.
Also known is apparatus for comparing small manufactured articles with a standard article to detect flaws. The apparatus has an optical system (basically a binocular microscope) through which an observer can view selectively either the standard article or a test article, and can switch between the two of them. If the two articles are arranged so that the observer receives images of them which are substantially identical (in position, size, brightness and so on), then the flaws may be quite easily detected. But it is extremely time-consuming to set up the apparatus so that the two images sufficiently approach identity of position etc.